OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
93 
perfectly developed parenchymatous medulla; whilst the four fragments, a a, of the 
primary central tracheseal axis of fig. 10 are still further separated from one another. 
Fig. 13 is an outline sketch of the central part of a Lyginodendvon Oldhamiurn, 
very advanced in growth, but only enlarged less than eight diameters instead of 
thirty, as is the case with figs. 10, 11, and 12. 
The line h represents the inner border of the exogenous cylinder. The five points, 
a , a , are the five dissociated segments of the originally solid tracheseal bundle, corre¬ 
sponding to those af a' of figs. 11 and 12. On making accurate measurements, I find 
that whilst the outer surfaces of these five bundles continue to be in the closest 
contact with seventy-one parts of the circumferential line h, the intervening spaces, 
a" a", non-existent in the young state of the branch, occupy 1(34 parts: so that Avhilst, 
in the first instance, the circumference of the circle h and tliat of the aggregate of 
the united bundles, a a, were absolutely coincident, the circle has enlarged, at the 
least, to more than three times its original circumference. Such an enlargement was 
necessarily accompanied by a corresponding increase in either the number, or size, 
or both, of the tracheseal laminse and their intervening medullary rays, constituting 
the exogenous xylem zone. 
Some of my botanical fiiends have endeavoured to explain the corresj^onding 
phenomena amongst the Lepidodendra, by supposing that, wdrere such differences 
exist in the size of the medullse, they did not represent different stages of groAvth 
of similar branches, but were characteristics of different kinds of branches; but 
no such improbable hypothesis is for a moment admissible in tlie case of this 
Lyginodendron. The effects of growth, and of growdh alone, as described above, are 
seen in every one of the innumerable specimens of Lyginodendron that have yet been 
discovered. In this case, that these are the effects of groAvth, is seen, first, in the 
breaking up, by dissociation, of the solid axial bundle of tracheae ; secondly, in the 
formation of an area in the centre of this bundle, which became occupied by a 
steadily expanding parenchymatous medulla, and, lastly, in the secondary effects of 
these central expansions on the tracheseal xylem and the cortical zones which enclose 
the medulla. To dispute this is, in my opinion, alike unscientific and futile. 
When wm note that the medulla of the largest stem of T^yginodendron which I haA'e 
yet obtained has advanced from little more than a point to above an inch in mean 
diameter, it is clear that sucli an enlargement must have been accompanied by con¬ 
siderable changes in the relations of the more external tissues, however ajAparently 
permanent they might be. 
In order to obtain some fairly accurate data in reference to this strain put upon 
these more external structures, I have counted as accurately as the subject admitted 
of, the number of the radiating laminse of the trachese comprising the xylem zone 
which started from the inner margin of that zone, and also the number of those Avhich 
reach its outer or cortical zone. The results of these observations made on sixteen 
